In many semiconductor wafer processing systems wafers undergo treatments, such as annealing or deposition, by subjecting them to a flow of processing gas. Especially when the wafer is floatingly supported by such a gas flow, without further mechanical constraints, positioning the wafer within a few hundred micrometers from a desired position near a processing body is a prudent task that needs to be executed carefully. The accurate positioning of the wafer is required for reliable system performance and a repeatable and high quality processing result, and implies accurate control over the distance between the processing body and the wafer. For such accurate control, some type of measurement of the distance between the processing body and the wafer would be highly desirable.
Distance determination involving optical measurements is inflicted with the difficulty of a variable reflectivity of the wafer, depending on the combination of films of dielectric and conductive materials that have been deposited on the wafers. Some combinations of films may be highly reflective whereas other combinations may be highly anti-reflective. Further, mechanical measurements are difficult to implement in complex semiconductor equipment and in many cases mechanical contact with the wafer is not desirable and/or acceptable.
In one embodiment, it is an object of the present disclosure to provide a method for accurately determining a change in a distance between a processing body and a wafer. In another embodiment, it is an object of the present disclosure to provide a method for accurately monitoring the alignment of a wafer handler relative to a processing body. In yet another embodiment, it is an object to provide a method for detecting variations in the thickness of a substantially flat wafer, which variations may influence the position of the wafer relative to a processing body.